Gelatins and chemically modified gelatins are widely used for a variety of commercial purposes. Gel formation is essential to almost all commercial applications involving gelatin, occurring when the aqueous solutions prepared under moderate heat are cooled. A gelatin normally counts as "good" when the melting point of the gel, for a given concentration, is relatively high and when the gel is mechanically solid. However, there are applications where gelation is either unnecessary or even undesirable. One example of this type of application is the use of the gelatin as a colloid in plasma substitutes or transfusion solutions of the kind used clinically, for example, for treating shock. Solutions of this kind, containing approximately 3 to 5% of gelatin, should remain liquid at temperatures down to around 0.degree.C. An approximately 4% solution of a standard commercial-grade gelatin with an average molecular weight (M.sub.n) of from 30,000 to 70,000 gels at temperatures of from 24.degree. to 32.degree.C and, for this reason, is not suitable for direct use as a colloid in a blood plasma substitute.
Although the gel-melting point can be reduced to almost any extent by hydrolytically degrading gelatin, i.e. by reducing the average molecular weight, this does not apply to gelatin of the kind used for transfusion purposes, because the colloid leaves the circuit more quickly, the smaller the molecules.
Accordingly, attempts have been made to modify gelatin chemically in such a way that gelability (as measured from the gel-melting point) is drastically reduced for a given average molecular weight. In this connection, it is known that gelatin can be simultaneously degraded and crosslinked, branched or, in some cases, even inner-molecularly bridged molecules can be formed from the linear gelatin molecules. Although the average molecular weight can be kept at a relatively high level, the average free (unbranched) chain length becomes shorter. This complicates reformation of the helical conformation typical of collagen and gelatin which is a prerequisite for gelatin.